The present invention relates in general to switches, and more particularly to a switch used as a range selector switch in an ion chamber type instrument for measuring radioactivity.
Ion chamber instruments are well known in the art of detecting and measuring beta, gamma, and x-ray radiation. Examples of such instruments are a Model RO-2 Ion Chamber Instrument sold by Eberline Instrument Corporation of Sante Fe, New Mex., and a Model RSO-50 Ion Chamber Survey Meter sold by the assignee of the present invention.
In these prior art instruments, an ion chamber, operating in the current mode, provides a very small direct current to the input of a very high gain operational amplifier configured as a current-to-voltage converter, the current gain of the operational amplifier being determined by a portion of the amplifier output current fed back into its input. The output of the amplifier drives a meter which indicates the amount of radiation per unit of time impinging on the ion chamber. The ion chamber, amplifier and meter constitute the basic components of a radiation measuring circuit well known in the art.
To change the range of operation of the measuring circuit, the current gain of the operational amplifier is changed. This gain change is accomplished by use of a resistor network including range selector switches, feedback current from the amplifier output being fed back to the amplifier input via the resistor network. By opening and closing the range selector switches, the user can vary the resistance of the network, and thus the amplifier current gain so as to set the measuring circuit at a desired operating range.
Because of the very high current gain of the amplifier and the very small current from the ion chamber that is being measured, the range selector switches, when in an "off" or non-conducting condition, must have a very high "off" resistance, typically on the order of 1.times.10.sup.15 ohms or more, to minimize error causing, leakage current feedback into the input of the operational amplifier. Conventional mechanical switches, while capable of providing the required high "off" resistance, can become dirty so as to inject undersirable noise or leakage currents into the measuring circuit. In place of conventional mechanical switches, hermetically sealed magnetic reed switches have been successfully used as range selector switches in both of the above-noted prior art ion chamber instruments. To actuate the magnetic reed switches, a mechanical linkage is used to move magnets toward and away from the reed switches. While such mechanically actuated, magnetic reed switches have been successfully used as range selector switches in ion chamber instruments, they are still, because of their mechanical nature, costly, and are prone to failure. Also, spurious magnetic fields can cause the reed switches to inadvertently actuate leading to measurement errors.
While it may seem appropriate to one skilled in the art to substitute a semiconductor switch, such as a transistor, in place of the above-noted magnetic reed switch, current state of the art transistors, when in an "off" or non-conducting condition, do not, by themselves, have a high enough "off" resistance (typically only 1.times.10.sup.13 ohms at best) to limit leakage feedback current to the required minimum levels.